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Electrical Stress Induced Structural Dynamics in Silicon Oxide Resistive Memories

Patel, Kamal; (2021) Electrical Stress Induced Structural Dynamics in Silicon Oxide Resistive Memories. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

In this thesis, the effects of electrical stress on silicon oxide resistive random access memory (RRAM) devices are studied with a view of understanding the individual mechanisms involved in RRAM operation. This is achieved through a combination of density functional theory (DFT) modelling and characterisation using transmission electron microscopy (TEM). In Part I of the thesis, DFT is used to model the incorporation, diffusion, reduction, and cluster nucleation of Ag in Ag/SiO2/Pt RRAM devices. It is found that Ag incorporates into SiO2 as a Ag+1 ion, which is mobile through large rings, grain boundaries and column boundaries. An O vacancy (VO) mediated Ag cluster model is then proposed, where Ag+1 reduction is shown to occur at 33% and 11% of VO sites at the Ag and Pt electrodes, respectively. In this case, Ag+1 ions bind to VO forming the [Agi/VO] j complex, which is favoured to trap electrons from the electrodes. In this way, as a Ag cluster grows, the metallic Ag-Ag bonding compensates strain in the lattice leading to the breaking of Si-O bonds. The broken Si-O bonds open access to new voids into which small Ag clusters may break from the original Ag cluster and form, providing new sites for cluster nucleation. In Part II of the thesis, Au-Ti-SiOx-Mo, Au-SiOx-Mo and Ti-SiOx-Mo (x approx 1.95) RRAM devices are characterised through TEM. It is shown the roughness of the Mo layer leads to patterning in the device, where voids and column boundaries form in SiOx at the troughs of the SiOx/Mo interface. The column boundaries are shown to facilitate the transport of Ti and Mo during positive electroforming leading to conductive metal-oxide filaments in the SiOx layer. Conversely, oxygen is dispelled from SiOx under negative electroforming, allowing electron tunneling via trap assisted tunnelling through VO sites.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Electrical Stress Induced Structural Dynamics in Silicon Oxide Resistive Memories
Event: UCL (University College London)
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2021. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Electronic and Electrical Eng
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10123271
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